1. Field of the Invention
The present disclosure relates to a developing apparatus that develops an electrostatic latent image formed on an image carrier by an electro-photographic method or an electrostatic recording method, and particularly to a developing apparatus of a two-component developer type including tonner and carrier.
2. Description of the Related Art
An image forming apparatus such as a copy machine using an electro-photographic method visualizes an image by applying toner to the electrostatic latent image formed on an image carrier such as a photosensitive drum. Such a conventional developing apparatus is known that uses two-component developer (hereinafter, referred to as “developer”) including toner and carrier, and includes a first conveyance screw and a second conveyance screw that convey the developer as stirring it. The first conveyance screw provided in a developing chamber is used to supply the developer to a developing sleeve, which is a developer carrier. The second conveyance screw provided in a stirring chamber is used to mix and stir the developer collected from the developing sleeve and the developer newly supplied. Such first and second conveyance screws are paired and convey the developer in opposite directions each other to circulate the developer between the developing chamber and the stirring chamber.
The developer is unevenly distributed in the developer container of such a developing apparatus. The developing apparatus of a “vertical, stirring type” will be described herein as an example. The developer container in the developing apparatus includes the developing chamber and the stirring chamber, which are separated from each other. The developing chamber supplies the developer to the developing sleeve, and the stirring chamber collects and stirs the developer from the developing sleeve. The developing chamber and the stirring chamber are disposed in a vertical direction, and a part of the developer conveyed from the developing chamber to the stirring chamber passes through a connection portion for connecting the developing chamber and the stirring chamber, and moves downward or upward.
During circulation of the developer at this point, the whole developer sent from the stirring chamber to the developing chamber does not reach a downstream end of the first conveyance screw in the developing chamber but some developer is supplied to the developing sleeve on the way. Such developer is collected into the stirring chamber after the developer passes through a development region. The developer is sent to the developing sleeve almost over the whole region in a longitudinal direction of the developing sleeve. Therefore, in the developing chamber, an amount of the developer conveyed by the first conveyance screw tends to gradually decreases it flows from an upstream end to the downstream end.
On the other hand, in the stirring chamber, the amount of the developer conveyed by the second conveyance screw tends to gradually increase as it flows from the upstream end to the downstream end. In other words, the developer is unevenly distributed in the developing apparatus.
The developer is unevenly distributed in the developer container as described above, which may cause image defect such as a white spot and density unevenness. The white spot is a part of an image portion which is not developed due to lack of the developer at a downstream side in a conveyance direction (hereafter, referred to as a “downstream side”) of the developing chamber for supplying the developer to the developing sleeve. The density unevenness is caused by insufficient supply of the developer.
Japanese Patent Application Laid-Open No 11-84874 discusses a configuration in which the uneven distribution of the developer is reduced by varying a conveyance capacity of the conveyance screws between an upstream side in a conveyance direction (hereafter, referred to as an “upstream side”) and a downstream side therein, in other words, by raising the conveyance capacity at the upstream side compared with at the downstream side.
However, if images are continuously output at a low printing ratio, the developer decreases its flowability since the developing becomes deteriorated. Particularly, the flowability is decreased when the toner is badly deteriorated, for example, due to removal of external additives from the toner. Thus, a speed for conveying the developer becomes slower in the developer container compared with when it is not deteriorated.
On the other hand, the decrease amount of the developer decreased per unit of time as it flows from the upstream end to the downstream end in the developing chamber, in other words, the amount of the developer conveyed by the developing sleeve, does not vary between before and after the toner deterioration. Namely, the decrease amount of the developer remains substantially constant.
Therefore, even if the uneven distribution of the developer can be reduced in the developer container in an initial state, in which the developer is not yet deteriorated, when the developer is deteriorated and decreases its flowability, the uneven distribution of the developer cannot be sufficiently reduced. In other words, when the developer is further deteriorated and decreases its flowability, the decreased flowability causes the uneven distribution on a surface of the developer.
Therefore, at the downstream side in a developer conveyance direction in the developing chamber for supplying the developer to the developer carrier, the image defect such as the white spot and the density unevenness due to the lack of the developer may occur.